1. Technical Field
The present disclosure generally relates to electronic circuits and, more specifically, to electromagnetic transponders equipped with a battery.
A field of application of the present invention relates to so-called NFC (Near Field Communication) devices, which are most often mobile telecommunication devices equipped with near-field communication circuits.
2. Description of the Related Art
Most devices provided with a battery and with a near-field communication router are designed to be powered by the battery when available or to be remotely supplied by extracting power from an electromagnetic field radiated by a terminal in the absence of a battery or when said battery is not available or discharged.
A transmission from a terminal to a transponder is generally performed in amplitude modulation of a high-frequency carrier (for example, at 13.56 MHz). Since the carrier must be able to supply power to the transponder when it is not powered by its battery, the modulation index is generally smaller than 100% so that the carrier is always present.
The modulation index is defined as being the ratio of the variation of the message amplitude to the variation of the carrier amplitude. In practice, the modulation index generally is 0.8, which means that the difference between the respective amplitudes of a high state and of a low state does not exceed 20% of the carrier amplitude. This modulation index conditions the sensitivity necessary for the demodulator on the transponder side so that said transponder can make out high-state transmissions from low-state transmissions.
Problems may arise when the charge of the transponder battery is at a level such that the transponder attempts to draw power from the electromagnetic field during high levels of the transmission and to draw power from the battery during low levels. In such a case, the modulation index is no longer respected, which may cause signal interpretation errors.